Lignin functionalization strategies and the potential applications of its derivatives – A Review

Abstract

Lignin is one of the most important and widespread carbon sources on Earth. Significant amounts of lignin are delivered to the market by pulp mills and biorefineries, and there have been many efforts to develop routes for its valorization. Over the years, lignin has been used to produce biobased chemicals, materials, and advanced biofuels on the basis of its variable functionalities and physicochemical properties. Today, lignin’s applications are still limited by its heterogeneity, variability, and low reactivity. Thus, modification technologies have been developed to allow lignin to be suitable for a wider range of attractive industrial applications. The most common modifications used for this purpose include amination, methylation, demethylation, phenolation, sulfomethylation, oxyalkylation, acylation or esterification, epoxidation, phosphorylation, nitration, and sulfonation. This article reviews the chemistry involved in these lignin modification technologies, discussing their effect on the finished product while presenting some market perspectives and future outlook to utilize lignin in sustainable biorefineries.

GRAPHICAL ABSTRACT

INTRODUCTION

Since pulping and paper industries were established at the end of the nineteenth century, a massive amount of residual lignin has been incinerated to recover chemicals and produce power and steam at the pulp mill sites. Such processing recovers only the fuel-equivalent value of the lignin. However, lignin may be a promising source of biobased materials such as polyols, antioxidants, binders, coatings, reinforcing agents, and biofuels. Besides being one of the most important and widespread carbon source on Earth, lignin is capable of delivering renewable aromatic compounds on an extensive scale (Bajwa et al. 2019). Pulp mills demonstrate an increasing interest in alternative lignin applications to diversify their product portfolio and generate additional sources of revenue. Other drivers to this end are improvements in pulping efficiency, the development of cost-effective technologies for lignin isolation, and the possibility of using underutilized biomass such as branches and bark for cogeneration. With this, in the mid-to-long term, more lignin will be made available for conversion to biobased fuels, chemicals and materials (Dessbesell et al. 2020).

Lignin is a natural macromolecule deposited in the plant cell wall as a result of condensation reactions involving phenylpropanoid radicals generated from p-coumaryl, coniferyl, and sinapyl alcohols during the lignification process (Figs. 1A, B). These cinnamic alcohols are the basis of the primary lignin structure, forming monolignols that are named p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units, respectively. These are distributed in variable ratios in the plant kingdom (Fig. 1C).

oxidization step that improves lignin filterability, reduces chemical consumption and minimizes the emission of volatile sulfur derivatives throughout the production process (Kouisni et al. 2016).

Excellent reviews can be found in the literature containing detailed information about pulping and pretreatment processes, lignin chemistry, and the prospects for use of technical lignins (Li and Takkellapati 2018; Schutyser et al. 2018; Bajwa et al. 2019).

LIGNIN FUNCTIONALIZATION

The presence of multiple functional groups represents the level of lignin’s complexity, besides its macromolecular structure. Functional groups such as alcohols, phenols, and ethers are distributed throughout the phenylpropanoid structure, decorating the biopolymer and making it susceptible to chemical modification (Fig. 2).

Fig. 2. Reactive sites of a lignin model structure and reactions that may be used to develop ideal properties for specific applications

Fig. 3. Main lignin transformations in aromatic and aliphatic (side-chain) moieties

Likewise, the original functional groups themselves may act as acids, nucleophiles, or electrophiles against a series of organic compounds. As a result, many transformations in lignin structure are possible, providing an increase or decrease in molecular mass distribution, an increase in reactivity due to the augmentation of highly reactive functional groups (e.g., phenolic and aliphatic hydroxyls), and an improvement of its compatibility with some synthetic polymeric matrices (Fig. 3). These include amination, methylation, demethylation, phenolation, sulfomethylation, oxyalkylation, acylation or esterification, epoxidation, phosphorylation, nitration, and sulfonation. Besides, improved solubility and thermal stability may be achieved, depending on the new functional groups attached to the lignin macromolecule.

Amination

Nitrogen is not an abundant element in lignin composition, as demonstrated by its elemental analysis (Wang et al. 2014; Ge et al. 2015). However, the use of amines to graft nitrogen into the lignin structure seems to be an exciting option for adding reactive sites capable of being used as intermediaries for lignin functionalization. In acidic conditions, amino groups are ionizable and positively charged, making lignin highly reactive in aqueous media (Wang et al. 2018a). Amino groups may also convert hydrophobic technical lignins such as kraft lignin into a highly hydrophilic material, improving its foamability, emulsifying properties, aging resistance, and mechanical strength (Liu et al. 2016). Also, aminated lignins may be used to remove dyes and heavy metals from aqueous systems due to their cationic and anionic absorbing capacities (Wang et al. 2014; Xu et al. 2017; An et al. 2020). Several studies related to the synthesis and applications of aminated lignin are listed in Table 1.

Table 1. Reactional Conditions for Lignin Amination and Potential Applications of the Final Products

Table 2. Reactional Conditions for Lignin Methylation and Potential Applications of the Resulting Materials

Methyl groups can be introduced into the lignin structure by nucleophilic aromatic substitution, bimolecular alkyl cleavage nucleophilic substitution, and bimolecular acyl cleavage nucleophilic substitution mechanisms depending on the reactants and reaction conditions (Sadeghifar et al. 2012; Sen et al. 2015). Methylation is more selective for phenolic hydroxyl groups because they are more acidic than aliphatic hydroxyl groups. As a stronger nucleophile, phenolate attacks the methylic carbon atoms in methylating agents, taking the methylic carbon atom for itself (Fig. 5).

Table 3. Reactional Conditions for Lignin Demethylation and Potential Applications of the Final Products

Chemical demethylation may be carried out with several reagents and reaction conditions, with sulfur compounds having a great potential to demethylate lignin (Hu et al. 2011). Li et al. (2016) tested several sulfur-demethylating reagents to prepare fast curing agents for phenolic resins. Among them, Na₂SO₃ produced demethylated lignin in 10 wt% aqueous NaOH at 90 °C for 1 h (Fig. 6).

Table 4. Phenolation of Lignin under Different Conditions

Jiang et al. (2018) conducted a more in-depth study about lignin phenolation with probable structures formed due to this reaction route. According to these authors, apart from breaking most of the ether bonds and secondary reactions, a significant decrease in aliphatic hydroxyl groups was observed due to the phenol incorporation in lignin substructures such as β-O-4′, β-5’/α-O-4′, β-β,’ α-carbonyl, and others. The phenolation process used phenol as a reactant and solvent, and lower acid concentrations (e.g., 5%) were used to improve the viability of the production process.

Taleb et al. (2020) studied both phenolation and acetylation of spent coffee ground lignin after pretreatment with dilute sulfuric acid. Compared to acetylation, phenolation led to more thermally stable (430 °C) lignin streams, having a higher availability of OH sites. Moreover, phenolated lignin had a superior adsorption performance of methylene blue (recovery of 99.6%), a compound used to evaluate the cationic adsorbent capacity of dyes. This study demonstrated a different application for lignin and offered a low-cost material to treat textile effluents. Although relatively inexpensive, the recovery and reuse of phenolated lignin were not demonstrated in this study.

Recently, phenolated Eucalyptus sp. alkaline-extracted lignin was studied as a matrix for cellulase adsorption. Lignoresorcinol (LigR) and lignopyrogallol (LigP) phenolated lignins (Fig. 7) had maximum cellulase adsorption capacities of 842.1 and 911.4 mg g^-1, respectively, compared to 76.5 mg g^-1 of the starting material. The enzymes were removed from both phenolated lignins by changing the pH from 10 to 4.8, with LigP adsorbing ten times more cellulases than lignin without modification (Mou et al. 2020). Compared to LigR, LigP provided a better enzyme migration to fresh cellulosic materials during the enzymatic hydrolysis stage, with LigP-desorbed enzymes displaying a higher total cellulase activity and a better hydrolysis performance.

Fig. 7. Structure of pyrogallol and resorcinol phenolated lignins

Thébault et al. (2020) evaluated the effects of lignin type and substitution degree on the properties of phenol-formaldehyde resins based on phenolated lignins. A factorial design was carried out using kraft and lignosulfonate lignins, unmodified and phenolated lignins, and level of phenolation (30 or 50%) as independent variables. Phenolation increased the number of reactive sites and decreased the average molecular mass of lignin. As a result, phenol-formaldehyde resins based on phenolated lignins had surface tension, viscosity, molecular mass distribution, and reactivity higher than those derived from unmodified lignins.

Wang et al. (2020) synthesized lignin-containing phenol-formaldehyde wood adhesives (LPF) from fractions of an industrial birch alkaline lignin that was previously submitted to sequential solvent extraction with isopropyl alcohol (i-PrOH), ethanol (EtOH), and methanol (MeOH). All these fractions were characterized to elucidate LPF structure/performance correlations. Carbohydrate, ash, and Klason lignin contents increased along this solvent extraction sequence and lignin apparent weight-average molecular mass (M_w). Lignin, phenol, and formaldehyde were used as reactants using a one-pot reaction system to produce LPF. Lignin was integrated covalently into the phenol-formaldehyde resin, and this was correlated to its adhesive strength. Also, the incorporation of lignin with high M_w and a high degree of condensation (MeOH lignin) affected the resin adhesiveness. This work was the first to demonstrate the feasibility of fractionating industrial birch alkaline technical lignins to produce thermoset LPF wood adhesives.

Depolymerization followed by phenolation was carried out by Zhou et al. (2020) to synthesize lignin-based phenolic foams. Under optimal reaction conditions, both M_w and number-average molecular mass (M_n) decreased from 12.600 and 6.480 g mol^-1 in alkaline lignin (AL) to 6.100 and 1.500 g mol^-1 in phenolated alkaline lignin (PAL). In addition, lignin phenolic hydroxyl groups increased from 2.4 in AL to 3.3 mmol g^-1 in PAL. Foams were also characterized for their physical, mechanical, thermal, and morphological properties. Both macro and micro images of PAL-based and AL-based phenolic foams revealed that the foam structure remained uniform for lignin incorporation up to 30%. Both samples had better thermal stability, lower volumetric water absorption, and lower slag rate than foams synthesized in the absence of lignin. Also, for materials with the same degree of substitution, PAL-based phenolic foams had higher compressive strength and a more uniform structure than AL-based foams.

Sulfomethylation

Low water solubility is a limiting factor for the valorization of kraft lignins. In this context, lignin sulfomethylation produces water-soluble derivatives by introducing a methylene sulfonate group in aromatic rings (Aro and Fatehi 2017). This process differs from sulfite pulping, whereby sulfonic acid groups are placed in the lignin aliphatic side-chains. Figure 8 shows that lignin sulfomethylation occurs by adding sodium sulfite anions into alkaline media, preferably at the unsubstituted C₅ of lignin substructures. This reaction takes place at various pH (7 to 13), temperatures (60 to 160 °C), reaction times (0.5 to 9 h), and sulfite/lignin and formaldehyde/lignin mass ratios of 0.1 to 1.0 and 0.01 to 1.0, respectively (Aro and Fatehi 2017; Eraghi Kazzaz et al. 2019; Konduri and Fatehi 2015). He and Fatehi (2015) studied the sulfomethylation of LignoForce^TM kraft lignins using formaldehyde (HCHO) and Na₂S₂O₅. The maximum estimated sulfonation degree was achieved at 97.1 °C for 3.2 h using 0.97/1 HCHO-to-lignin and 0.48/1 Na₂S₂O₅-to-lignin molar ratios.

Qin et al. (2015) investigated the use of grafted sulfonated alkali lignin (GSAL) as a dispersant for coal-water slurries. GSAL was synthesized by sulfomethylation followed by etherification and polycondensation to obtain high levels of sulfomethylation and high molecular mass distributions. In this application, sulfomethylation was carried out for 1 h at pH 10 and 60°C.

Fig. 8. Scheme of sulfomethylation of lignin with sodium sulfite (adapted from Eraghi Kazzaz et al. 2019)

Different conditions in lignin sulfomethylation allow the obtainment of sulfomethyl-derivatives with various degrees of substitution, leading to materials with different average molecular masses and degrees of sulfonation. This reaction has been applied to kraft lignins by the MeadWestvaco Corporation (now Ingevity) to produce dye dispersants marketed as Reax^® since the 70’s (Meister 2002). Sulfomethylated lignins (SML) have also been used as water reducer for cement admixtures (Kamoun et al. 2003), flocculant in water purification systems (Bolto and Gregory 2007), dispersant in pesticide formulations (Li and Ge 2011), and also as potential corrosion inhibitors for iron-based materials (Abu-Dalo et al. 2013).

Sulfomethylation side reactions include sodium thiosulfate formation, which may be avoided using high temperatures (100 to 150 °C) and sodium sulfite in excess to improve SML yields. Another hindrance is found in the lower reactivity of hardwood lignins because, unlike guaiacyl, the nucleophilic attack of phenolic hydroxyl groups is hindered by its high degree of methoxylation.

Konduri and Fatehi (2015) studied the sulfomethylation of hardwood kraft lignins. The main objective of the work was to produce water-soluble kraft lignin with an anionic charge. The optimal reaction conditions involved a lignin concentration of 20 g L^-1, 0.5 mol L^-1 of NaOH(aq), 0.9 sodium hydroxymethyl sulfonate/lignin molar ratio, 100 °C, and 3 h of reaction time. These conditions provided an SML with a charge density of −1.6 meq g^-1 and 1.48 mmol g^-1 sulfonate groups, while the unmodified lignin had a negligible charge density and 0.03 mmol g^-1 sulfonate groups. The SML solubility in water at neutral pH was successfully improved to 40 g L^-1, in contrast to the insolubility of the unmodified lignin. Further increments in the degree of sulfomethylation were not possible due to the formation of undesirable byproducts such as sodium thiosulfate. However, the density of sulfonate groups achieved in this study was higher than that obtained by Wu et al. (2012) after sulfomethylation of corn stalk alkaline lignin, reaching 1.29 mmol g^-1 as reported in the literature.

Huang et al. (2018) performed the sulfomethylation of alkaline lignin (AL), and enzymatic hydrolysis lignin (EHL) derived from alkali-pretreated bamboo fibers. SML yields of circa 95% were achieved from AL sulfomethylation after 3 h at 110 °C using a sodium hydroxymethylsulfonate/lignin molar ratio of 1.0. The maximum lignosulfonate yield from EHL was only 68.9% when the reaction was carried out for 4 h at 110 °C with a sodium hydroxymethylsulfonate/lignin molar ratio of 0.8. The largest sulfomethylation of AL was attributed to the availability of more reaction sites (free C5 position of lignin), possibly due to its lower average molecular mass and the presence of residual carbohydrates in EHL.

One additional drawback of sulfomethylation is the use of formaldehyde, which is carcinogenic, mutagenic, and environmentally unfriendly. Hence, recent studies are focused on developing alternative sulfoalkylation reaction routes to alleviate its environmental impact. For instance, lignin sulfobutylation provides water-soluble lignin derivatives similar to conventional sulfomethylation with the advantage of being carried out in aqueous media (Eraghi Kazzaz et al. 2019; Hopa and Fatehi 2020; Huang et al. 2018).

Fig. 9. Sulfobutylation of phenolic and aliphatic hydroxyl groups in lignin with 1,4–butane sultone (adapted from Hopa and Fatehi 2020)

Sulfobutylation is a one-step reaction in which 1,4–butane sultone reacts with both phenolic and aliphatic hydroxyl groups of lignin (Fig. 9). In general, sulfobutylated lignin (SBL) is produced at 70 °C for 6 to 7 h at pH 12. Although sulfobutylation requires temperature and pressure lower than sulfomethylation, the sulfobutylation reagent is more expensive. Also, sulfobutylation leads to a higher degree of substitution because sulfomethylation is limited to lignin phenolic hydroxyl groups (Eraghi Kazzaz et al. 2019; Hopa and Fatehi 2020).

Hopa and Fatehi (2020) compared both sulfobutylation and sulfomethylation of a softwood kraft lignin to produce sulfoalkylated lignins with similar charge densities (circa -2.3 meq g^-1). Both products were effective as kaolin dispersants at neutral pH. One advantage of sulfobutylation was that the quantity of reagents to achieve similar charge densities was lower than that of sulfomethylation. The required 1,4-butane sultone/lignin molar ratio for SBL was 0.2, while an equivalent charge density in SML was only achieved using a formaldehyde/lignin molar ratio of 2 and a sodium metabisulfite/lignin molar ratio of 1. These findings showed the possibility of obtaining sulfoalkylated lignin derivatives with similar properties under more environmentally friendly conditions.

Oxyalkylation

Lignin has a high potential to react through oxyalkylation with reagents such as ethylene oxide, propylene oxide, and others (Cateto et al. 2009; Wu and Glasser 1984). Particularly, oxyalkylation has been recognized for overcoming lignin’s poor solubility and its frequently observed adverse effects on mechanical properties of solid polymers, viscosity, and cure rate of resin systems (Wu and Glasser 1984). Chemical modification by oxyalkylation has been demonstrated to offer a route to improve solubility, to reduce the brittleness of lignin-derived polymers, and to upgrade viscoelastic properties in various uses, such as prepolymer for engineering plastics (Li and Ragauskas 2012), for polyurethane films (Saraf and Glasser 1984), and rigid polyurethane foams (Zhang et al. 2019b). Also, oxyalkylation shows that all phenolic hydroxyl groups can fully be converted into lignin-polyether chains with hydroxyl groups, increasing the reactivity by reducing the sterically hindered phenolic OHs (Kühnel et al. 2017; Zhang et al. 2019b).

Lignin oxyalkylation may result in a copolymer combining covalently high modulus lignin with a lower modulus aliphatic polyether phase. In the industry of polymers, this reaction is a comprehensive method for the production of polyols as precursors for the polyurethane production, especially with propylene oxide (PO), since the procedure provides a chain extension reaction forming grafts of poly(propylene oxide) (Kühnel et al. 2017; Zhang et al. 2019b).

The oxyalkylation of lignin with epoxides enables the substitution of conventional petrochemical polyols by a renewable polyol source (lignin) and produces interesting macromonomers for polyurethane synthesis, namely bio polyols. This is interesting since polyurethanes are used in many industries due to their wide-ranging mechanical properties and their ability to be relatively quickly processed in various forms (flexible and rigid foams, elastomers, adhesives, etc.) (Berrima et al. 2016; Kühnel et al. 2017).

The reaction of lignin with PO is an etherification that requires a catalyst, commonly alkaline (KOH), but also acidic catalysts such as H₃BO₃, Al₂O₃, AlCl₃, ZnO can be employed (Fig. 10). The reaction is usually carried out at high temperatures (>150 °C) and high pressure (>10 bar). The resulting products from oxypropylation are a mixture of oxyalkylated-lignin and poly (propylene oxide) (PPO) diols, and the last arises from the homopolymerization of PO. Both materials were already applied to synthesize rigid polyurethane foams (Wu and Glasser 1984; Lee et al. 2017; Zhang et al. 2019b).

Fig. 10. Scheme of oxypropylation of lignin for bio polyol production; adapted from Zhang et al. (2019b)

The pioneering investigation on oxypropylation of lignins was carried out by Wu and Glasser (1984). PO was employed in alkaline conditions (KOH) at 180 °C for the hydroxypropylation of lignin and lignin-like model compounds. It was observed that both homopolymerization and copolymerization occurred during batchwise hydroxyl-propylation of lignin-like model compounds and lignin. There was a dependence between the increase of reaction rate and the increasing KOH concentration until 2.6 mmol mol^-1 PO. Also, the retardation effect of functionalization on the initial rate of PO homopolymerization decreased in the order of COOH > phenolic OH > aliphatic OH. Interestingly, the authors showed that the volume of the solvent used in the reaction could be as low as 1 to 1.5 mL PO g^-1, which is advantageous, since the solvent used was toluene, a toxic, flammable, and volatile compound. Several of these findings, such as the effect of catalyst concentration on reaction rates, optimal temperature, and product viscosity, were used in subsequent investigations regarding oxypropylation.

Lee et al. (2017) performed the oxypropylation of methanol-insoluble softwood kraft lignin at 40 °C and 1 atm for 12 h in the presence of NaOH. Afterward, the modified kraft lignin was reacted with sebacic acid or polybutadiene (dicarboxy terminated) to produce monomers for polyester synthesis. The use of mild reaction conditions in the absence of organic solvents was claimed beneficial for the process economics and for the environment. The polyesters prepared in this way presented a higher decomposition temperature compared to other lignin-based polyesters. The temperatures were enhanced from 217 °C and 367 °C (oxypropylated lignin) to 380 °C and 453 °C for polyesters derived from sebacic acid and polybutadiene, respectively. Despite these advantages, the synthesis of polyols with propylene oxide is discouraged because PO is a harmful chemical due to its high vapor pressure, explosiveness, flammability, toxicity, and carcinogenicity (Kühnel et al. 2017).

Table 5. Lignin Esterification Routes and Some Applications

Table 6. Main Lignin Epoxidation Routes and their Applications

Table 7. Main Lignin Phosphorylation Routes According to the Literature

Prieur et al. (2016) phosphorylated kraft lignin to be added as a flame retardant of acrylonitrile butadiene styrene (ABS). Phosphorus grafting was achieved by dissolving lignin in THF with the addition of P₂O₅. The reaction was performed under reflux for 7 to 8 h, by which time phosphorylated lignin was largely precipitated. Water was added to inactivate the phosphorylating agent, and THF was removed by evaporation, resulting in the incorporation of 4.0 wt% of phosphorus in lignin. Incorporation of 30 wt% of this modified lignin into ABS resulted in composite with a significant reduction of its heat release rate. Also, phosphorylation increased the thermal stability of kraft lignin by 20 wt%. The authors claimed that this reaction system could be scaled up for future industrialization.

Gao et al. (2020) phosphorylated lignin in an aqueous medium containing urea and ammonium dihydrogen phosphate as the phosphorylating reagent. The mixture was reacted at 70 °C for 60 min with constant stirring. Next, lignin was dried at 70 °C and cured at 150 °C for 4 h. The loosely attached chemicals after the curing step were removed using cold deionized water. Phosphorylation by nucleophilic substitution of phenolic and aliphatic hydroxyl groups was confirmed by ¹H NMR, ³¹P NMR, and X-ray photoelectron spectroscopy (XPS). The resulting material exhibited a maximum decomposition temperature of 620 °C compared to 541 °C of the unmodified lignin.

Matsushita et al. (2017) synthesized an advanced biobased flame retardant from kraft lignin by introducing an intumescent group (3-amino-1,2,4-triazole) into the lignin structure followed by phosphorylation with phosphoryl chloride for 1 h at room temperature. Afterward, phosphoryl end groups were crosslinked using several alkane diols in pyridine, with the final products being purified by washing with water and methanol. Phosphorous incorporation was relatively high in the range of 6 to 10%, and this was directly related to the quantity of phosphoryl chloride and the chain length of alkane diol used. In simple combustion tests using a gas burner at 1300 °C, the resins exhibited non-burnable properties, producing only a swollen char. Also, the peak heat release rate was significantly lower than that of a commercial phenol-formaldehyde resin.

Costes et al. (2016) performed a two-step phosphorus/nitrogen grafting of kraft and organosolv lignins to be used as a flame retardant additive in poly(lactic acid) (PLA). For the first step, lignin was solubilized in chloroform and heated to 50 °C, then POCl₃ was added dropwise. The reaction continued for 17 h with constant stirring at 60 °C. The product was recovered by filtration and washed with fresh chloroform to remove the POCl₃ excess. Lignin was dried at 60 °C in a vacuum oven. In the second step, nitrogen was added by treating the modified lignin with aqueous NH₄OH at room temperature for 2 h. The final products were obtained by washing, filtration, and drying. Both kraft and organosolv modified lignins incorporated about 9% of phosphorous and 7% of nitrogen. The addition of 20 wt% of these lignins to PLA reduced its heat release during combustion owing to the formation of an insulating char layer at the surface of the burning sample.

Nada and Hassan (2003) phosphorylated cotton stalks kraft lignin and applied it as an adsorbent for heavy metal ions. Cotton stalks, anthraquinone brown stocks, and bleached pulps were also used for comparison. Phosphorus was grafted in all fractions using phosphorus oxychloride as the phosphorylation agent. Reactions were carried out at 115 °C for 2 h in a pyridine and dichloromethane mixture, and products were obtained after filtration, washing with dilute HCl (0.1 mol L^-1), and distilled water, and drying. Lignin fraction was shown to incorporate a phosphorus content (33.5 mg g^-1) higher than that of the other cellulosic fractions. All modified materials exhibited high adsorption capacities for Cr and Cu ions (~1 mg g^-1). The authors claimed that phosphorylated lignin had the highest adsorption capacity, but the results do not appear to have been statistically significant.

Nitration

Lignin nitration is a simple process that is usually performed using non-aqueous solvents. The most widely used nitrating agents are nitric acid in concentrated acetic acid, acetic anhydride, or sulfuric acid (Meister 2002). The resulting nitrolignin (NL) is an amorphous powder, ranging in color from yellow to brown with an average molecular mass between 600 to 2000 Da. Nitration causes an extensive lignin dealkylation, and nitrogen incorporation is generally around 7 wt% (Zhang and Huang 2001; Meister 2002).

Lakhmanov et al. (2020) performed the nitration of lignin with nitric acid in binary water-aprotic solvent mixtures. Acetyl nitrate, a mixed anhydride of nitric and acetic acids, was also used as the nitrating agent. When nitration was performed with nitric acid, the maximum yield of nitrated lignin (83 to 101%) was achieved using 1,4-dioxane, acetonitrile, and tetrahydrofuran, while the maximum nitrogen content (4.3 to 4.5%) was obtained using 1,4-dioxane or acetonitrile. The use of DMSO and DMF showed product yields of 23 to 35% and nitrogen contents of 1.3 to 3.9%, but their high oxygen contents were indicative of depolymerization and some oxidative transformations. The nitration with acetyl nitrate, on the other hand, formed the product with up to 4.7% nitrogen content.

NL has been identified as a promising candidate for electrochemical applications. For example, nitrogen-doped carbons derived from NL were used for the electrochemical reduction of oxygen, showing catalytic activities comparable to non-noble metal catalysts (Graglia et al. 2016). NL was used together with a castor oil-based polyurethane (PU) prepolymer to produce a polymer network. Covalent bonds were formed between the PU prepolymer and NL, forming a crosslinked material with good mechanical properties and high thermal stability. Also, NL showed a thermal reactivity with PU much higher than nitrocellulose (Zhang and Huang 2001).

Sulfonation

Sulfonation is a reaction whereby sulfonate groups are added to lignin using sulfuric acid or sodium sulfite as reagents (Meister 2002). The increase in the sulfur content in lignin correlates with an increase in its dispersion performance in suspensions (Lou et al. 2013). The kraft lignin sulfonation occurs in the α-position of phenylpropanoid units, which is commonly occupied by α-O-4′ aryl-ether and other ether bonds. Hence, the complete sulfonation of lignin is impaired by such ether bonds in its structure (Chakar and Ragauskas 2004; Crestini and Argyropoulos 2002).

Functionalization such as demethylation, phenolation, hydroxymethylation, reduction, oxidation, or hydrolysis increase kraft lignin reactivity toward sulfonation, yielding highly charged sulfonated lignins (Filley et al. 2002; Hu et al. 2011; Meister 2002). Among these, phenolation is more effective than reduction, oxidation, and hydrolysis to improve lignin reactivity (Alonso et al. 2005).

Kraft lignin can be sulfonated directly by reacting with >95% sulfuric acid at temperatures below 40 °C (Dilling 1991). Then, sodium carbonate is used for neutralization, precipitating the sulfonated lignin (SL) as its sodium salt. This process was reported to increase the presence of sulfonate groups kraft lignin by 2.5 mol mol^-1. However, the downstream processing for SL purification can be complicated due to its high solubility in water. Membrane filtration is an option for this purpose at the expense of a significant increase in operating costs.

Softwood kraft lignin was modified by phenolation followed by sulfonation with sulfuric acid or sodium sulfite to produce water-soluble lignin materials. Pretreatment by phenolation generated SL preparations with a high charge density of 3.12 meq g^-1 and high solubility in aqueous systems. Treatment of phenolated lignin with sodium sulfite generated sulfonated lignins with a lower charge density of 1.20 meq g^-1, mostly due to the partial etherification of the most reactive α-position. These lignins were successfully used as coagulating agents for dyes, removing up to 90% of ethyl violet from simulated solutions (Gao et al. 2019).

Sulfonated lignins produced under normal pressure and temperature conditions were 96.9% soluble in water at neutral pH. By adding 1.49 mmol g^-1 of sulfur in the lignin structure, sulfonation increased its negative zeta potential and improved its adsorption capacity of cement particles, leading to a better dispersion effect and homogeneity of the cement matrix (Ouyang et al. 2009).

CURRENT MARKET

Although kraft pulping is the largest lignin feedstock source worldwide, lignin from sulfite pulping is currently more commercially important because lignosulfonates have realistic applications, while kraft lignin is practically all burned for heating and power generation. Nevertheless, promising applications are opening up for kraft lignins due to the significant volumes produced worldwide.

Fig. 12. Lignin commercial brands and their applications by the prominent companies

Companies such as Ingevity, West Fraser, Domtar, RISE LignoDemo AB, WestRock, Suzano, and Liquid Lignin are dedicated to kraft lignin valorization. Likewise, Borregaard, Sappi Biotech GmbH, Domsjo Fabriker AB, Nippon Papers, and Rayonier Advanced Materials invest in technologies to produce lignosulfonates (Fig. 12). Together, these companies upgrade 1.8 million tons of lignin and lignin-derivatives per year, almost all at the commercial scale. Lignosulfonates supply ~79% of the lignin market (except for combustion), with minor contributions of kraft (~16%) and organosolv and hydrolysis (~5%) lignins (Dessbesell et al. 2020).

Borregaard (Sarpsborg, Norway), the major lignosulfonate producer worldwide, is present in four continents and delivers sustainable lignin-based materials, such as binding agents for animal feed, briquetting, dispersing agents for concrete, textile dyes, pesticides, battery components, and ceramic materials (Borregaard 2021).

After Borregaard, Domsjö Fabriker (Aditya Birla Group, Örnsköldsvik, Sweden), a prominent biorefinery facility, produces around 120,000 tons per year of powder lignin. Lignin is primarily sold for the external market to be used as an additive in concrete and feed industries, agriculture, and as a stabilizer for unpaved roads. Aditya Birla Group is a multinational business comprised of 50 companies with 130 production units in almost 40 countries (Domsjö 2021).

Sappi Biotech GmbH (Johannesburg, South Africa) developed technologies to apply lignosulfonates in leather tanning, dust control, recyclable packaging, water treatment as dispersing agents, and synthesis of phenolic resins. Besides, their lignosulfonates can be used as polyols and additives for ceramics, refractory materials, clay bricks, and bonding agents in fertilizers for soil improvements (SAPPI 2021).

Nippon Papers (Tokyo, Japan) takes advantage of lignin as dispersing, binding and chelating agents, selling around 70,000 tons per year of industrial brands such as SAN X^®, VANILLEX^®, and PEARLLEX^®. These are lignosulfonate-based products that interact with various polymeric matrixes due to their abundance of functional groups and structural versatility (Nippon Papers 2021).

Rayonier Advanced Materials (Shelton, USA) produces ARBO^TM for use as binding, chelating, and dispersants. Its lignin-based production is close to 130,000 tons per year in facilities located in the USA, France, and Canada (Rayonier, 2021). Also, both the Burgo Group (Verzuolo, Italy) and Shenyang Xingzhenghe Chemical (Shenyang, China) make the material available in the form of civil construction components. The latter company markets lignosulfonates in chrome, ferrochrome, potassium, calcium, and sodium forms. These materials have good properties for water reduction agents, adhesive, viscosity controllers, deflocculant, and fluid loss control agents (Shenyang Xingzhenghe Chemical 2021).

Concerning kraft lignin, Stora Enso (Helsinki, Finland), the oldest pulping company globally, started in 1288 as a copper mining company and diversified later to the forestry business. Today, it is the largest producer of commercial kraft lignin, with a production of circa 50,000 tons per year. Isolated by the LignoBoost process, this lignin branded as Lineo^TM is available with different dryness levels according to customer requirements for specific needs (Stora Enso 2021).

Ingevity (North Charleston, SC, USA), with an annual production of 40,000 tons per year, applies pine kraft lignin (Indulin AT) as the organic component in batteries. They also produce Kraftplex^®, a kraft sodium lignosulfonate formulated to ensure better negative plate expanders for batteries. Besides, Ingevity’s kraft lignosulfonates are used to manufacture dispersants and surfactants to be applied in asphalt paving, agrochemicals, adhesives, lubricants, inks, coatings, elastomers, and bioplastics (Ingevity 2021).

Companies such as WestRock (formerly MeadWestvaco, Atlanta, GE, USA), Domtar (Fort Mill, USA), and Suzano (São Paulo, Brazil) operate with lignin production capacities of 30,000, 25,000, and 20,000 tons per year, respectively. The former has supplied kraft lignin to the market since the middle of the twentieth century. In 2014 Domtar started the commercial production of Biochoice™, a high-quality lignin based on the LignoBoost process. In this case, applications range from natural binders to fuels, additives, coatings, plastics, resins, and other carbon-based materials (Domtar 2021; WestRock 2021).

Since 2013, Suzano, a Brazilian pulp and paper mill in line with lignin supply companies, produces Ecolig, a renewable technologic platform of lignin-based products (Ligseal, Ligflow, Ligform, Ligflex, Ligsperse). Such products are applied as antioxidants in rubber compounds, dispersing agents and additives for concrete, and building blocks for phenolic resins and plastics (Suzano 2021).

Lignin is only second to cellulose as the most available biopolymer on Earth, and yet it is still primarily underutilized, despite the wide variety of products described above. Indeed, lignin valorization is in its infancy; however, following the example of mature companies, lignin can be the most valuable resource in biorefineries to produce sustainable, innovative, and high-performance biofuels biobased chemicals, and biomaterials.

FUTURE OUTLOOK AND PERSPECTIVES

A saying is widespread among people from wood pulping backgrounds, that “One can make everything from lignin except money” (Ragauskas et al. 2014). This thought is based on the fact that scientists can do almost everything starting from lignin, but no company has yet been able to transform these technologies into profitable products on a large scale.

Although no market for profitable businesses using technical lignin yet exists, the future perspectives are outstanding in the short term. Wood pulping companies are investing in isolation processes such as LignoBoost^® and LignoForce^TM to increase the availability of good quality kraft lignin for the market. New and exciting applications have been developed in recent years based on the research of numerous scientists.

The first point to be considered is which of the available technical lignin products is being referred to when discussing possible industrial applications. Pulping companies produce around 100 million metric tons per year of kraft and other technical lignins worldwide. However, less than 2% of these lignins are commercialized, mainly in adhesives, surfactants, and dispersant formulations (Bajwa et al. 2019). Lignosulfonates are by far the most important lignin derivative, with a production of 1.3 million metric tons per year. Regarding kraft lignin, just 265,000 metric tons are isolated and used today for biorefining applications. Also, soda lignin plus hydrolysis lignin from cellulosic ethanol production, with 75,000 metric tons per year, are increasing in their importance and market share (Dessbesell et al. 2020). Hence, these lignin sources must be studied in detail to give options for the development of new profitable businesses. Moreover, with investment much more remarkable than many billions of dollars, kraft lignin is by far the most important to be considered for emergent economies and developed countries (Li and Takkellapati 2018).

Lignin is crucial for developing a biorefinery concept based on circular bioeconomy and attending to the basic definitions of green chemistry and chemical engineering. Albeit sustainability is the primary concern, the lack of economic feasibility is the bottleneck, which is aggravated by its heterogeneity and wide molecular mass distribution. Fractionation may overcome part of this technical barrier, as well as functionatization to make lignin more uniform and suitable for a wide range of possible applications.

Among future products made from lignin, carbon fiber, bio-oil, and phenolic resins are leading applications, but binders, adhesives, bioplastics, concrete admixtures, and biomedical applications are on the rise (Bajwa et al. 2019). In the future, the cost-effectiveness of high-volume bulk applications (energy, phenolic resins, PUs) will benefit from the development of high value-added products such as antitumor drugs and other specialties such as nanomaterials, antioxidants, and renewable hydrocarbons (BTX and advanced biofuels), among others.

PUs and phenolic resins are the most critical applications for technical lignins in the near future. However, in the long term, judging by the research and development efforts expended to date, carbon fibers, and depolymerization processes to apply lignin as a source for aromatic chemicals will play an important role (Li and Takkellapati 2018).

Even for PU and phenolic resin applications, barriers still exist to commercialize the production technologies (Lettner et al. 2020). For PU resins, these include the quality assurance to support large-scale operations, potentially lower production costs and sales prices, supply security, and proven environmental benefits. For phenolic resins, the list includes competitive costs compared to the current market, reliability of the production process, supply security, proven environmental benefits, and the need for integration along the entire value chain. The least impacting parameters are lignin price, environmental benefits, supply security, and the high technology readiness level of process operations from this latter list. Therefore, the application of lignin in phenolic resins should become a reality very shortly.

The price of lignin residues from acid and enzymatic hydrolysis will be driven by the market for electricity, with minimum values being estimated around US$ 18 to 37 and US$ 43 to 70 per lignin ton, respectively (Obydenkova et al. 2019). In the case of phenolic resins, one may envisage that the cost of fossil phenol should limit the maximum price for lignin. On the other hand, for some special lignin applications in which high purity and batch-to-batch reliability are required (e.g., carbon fiber manufacturing), price might be less important than quality and compositional repeatability.

ACKNOWLEDGMENTS

The authors are grateful to CNPq (grant 309506/2017-4) for providing financial support to carry out this study. This work was also financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001.

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